Water is a principal cause of distress and damage to paved structures such as roadways, airport runways and parking lots. Therefore, drainage systems are often provided in such structures in order to remove water from the paved surface or its foundations to thereby extend the useful life of the pavement surface. In some drainage methods, drainage systems are incorporated between the native soils or “subgrade” upon which a roadway or other large structure is situated and the overlying pavement surfaces. The present invention relates generally to synthetic void-maintaining structures with high permittivity and high transmissivity that are capable of extending the life of pavement by maintaining voids of sufficient dimensions to permit the timely egress of undesirable fluids, especially aqueous fluids.
In conventional road building, natural stone and aggregate materials are placed to form a drainable layer that is commonly called an Open Graded Base Course, or “OGBC.” OGBC's are typically used underneath the surfaces of highways, airport runways, roads, and parking lots that are paved with bituminous materials such as asphalt or cementitious materials such as concrete. The present invention provides a series of high-flow void-maintaining membrane laminate (“VMML's”) of polymeric material and related methods for economically manufacturing such laminates such that the need for an OGBC can be eliminated or minimized.
Pavement surfaces are highly engineered layered structures. Because of this, pavement structures require engineered materials that are selected based upon factors such as their density, particle or aggregate size, compressibility or other engineering parameters of the soil, stone and aggregate-based products that are required as structural fill that typically is installed in layers beneath pavement surfaces.
Two types of structural fill are the base course and, typically immediately beneath the base course, a sub-base course. Fluids such as water that become trapped or retained within structural fill cause damage to roadways and, over time, subsequently greatly reduce the useful life of a pavement system. These destructive phenomena occur even when asphalt additives, waterproofing techniques and conventional geosynthetics are used to improve the road.
The cause of many premature pavement failures has been traced to inadequate subsurface drainage. Typically, fluids enter the subsurface layers of pavement systems from surface infiltration through joints and cracks in the pavement, as well as pores in the pavement itself, seepage from the sides of the paved surface, and from rising groundwater beneath the road surface, either by capillary action or the upward movement of water in vapor form. In fact, the FHWA discovered that over 50% of all rainfall reaching a mature pavement surface enters underlying structural portions of the pavement through infiltration. In northern tier states, the destructive nature of water trapped in the structural base is exacerbated by freeze-thaw cycles, and particularly during spring thaw as ice lenses melt to create water-filled voids and very soft, water-saturated soils which lose a substantial amount of their compressive strength. In turn, these phenomena result in extensive damage to the highway system. These and related drainage-based structural issues are now well-recognized in the road and runway building industries.
When there is a high fluid content within soil or other layers supporting pavement that carries vehicular traffic, reduced bearing capacity can occur, resulting in deformation of the contour of the road surface, wheel rutting, and premature collapse or failure of the roadway. The American Association for Safety and Highway Transportation Officials (AASHTO) issued design methodologies in 1993 that underscore the observation that damage to roadways occurs when fluid such as water is retained. In promulgating standards for quantifying the drainage performance of highways and other paved surfaces, AASHTO rates pavement drainage performances from “excellent,” where water is removed from the roadway system within two hours, to “poor,” where water is removed within one month. Drainage coefficients corresponding to these ratings are often used as direct design parameters in highway construction. For example, the drainage coefficient corresponding to an “excellent” drainage system in a roadway section would typically be at least two times greater than the corresponding drainage coefficient for “poor” drainage system in a similar section of roadway. In general, a drainage system having a higher drainage coefficient increases the corresponding effective structural rating of a section of roadway. Therefore, higher drainage coefficients generally correspond to a longer or extended service life, or result in the reduction of the overall structural cross-section, and therefore the amount of engineered materials, necessary to support a particular load.
Other engineering parameters reflect the importance of sufficient drainage to roadways. For example, the presence of water in pavement causes a reduction of the resilient modulus, which reduces the ability of a pavement surface to support traffic loads. In 1993, AASHTO reported that water saturation can reduce the dry modulus of asphalt paving by 30% or more. Moreover, added moisture in unbound aggregate base and sub-base layers was estimated to result in a loss of stiffness on the order of 50% or more. With water retention, a modulus reduction of up to 30% can be expected for an asphalt-treated base as well as an increased erosion susceptibility of cement or lime-treated bases. In addition, with inadequate drainage, saturated fine-grain road-bed soil may experience modulus reductions of over 50%. Furthermore, the presence of fluids often causes the buildup of hydraulic pore pressure that, in turn, reduces the effective stress capacity of the soil materials that were placed to support the pavement system.
Premature failure of pavement systems results in unacceptably high life-cycle costs for highways and other large paved structures. One conventional approach to the prevention of such premature failure from occurring has been directed toward developing means and methods for waterproofing roads. After years of expense and effort, however, waterproofing paved surfaces sufficiently to extend their useful life has proven to be quite challenging and somewhat unsuccessful. At the present time, industry focus has shifted from attempts at preventing water from entering the pavement surface to developing ways for removing water from the subbase and other base materials underlying the pavement. This shift in focus has been the subject of a number of publications in the field. One such publication is Drainage of Highway and Airfield Pavements, H. R. Cedegren (1987, R. E. K. Publishing Co.). In his book, Cedegren emphasizes that proper base and subbase drainage are considered to be more essential than paved surface waterproofing with respect to assuring that a pavement structure will perform for the duration of its design life. Cedegren projects that pavement useful life can be extended up to three times (e.g., a service life can be extended from 15 years, to 45 years) if adequate subsurface drainage systems are installed and maintained. The benefits of good drainage are also recognized in many current roadway design methodologies published in the early 1990's by AASHTO and the U.S. Army.
Other published studies support this view. In one of them, “The Economic Impact of Pavement Subsurface Drainage,” R. A. Forsyth (1987, Transportation Research Record 1121, National Research Council, Washington, D.C.), the author reports at least a 33% increase in service life for asphalt pavement and a 50% increase for PCC pavements when subsurface drainage systems are used. Significantly, Forsyth observed a new crack reduction ratio of 2.4:1 when PCC pavements with subsurface drainage systems were compared to those without a subsurface drainage system. Moreover, other studies that reviewed pavements constructed to include base course layers constructed of non-uniform gradation, and consequently non-uniform and insufficient drainage capacity, concluded that service life was actually decreased by 50% when the pavement was saturated for periods as small as 10% of the year, that is, for approximately one month per year.
The economic disadvantages of inadequate subsurface drainage are significant. Indeed, KYDOT concluded that the costs of failing to properly drain a road could be up to $500,000 per mile when the costs of safety and repair delays are considered. KYDOT has also shown that providing a drainage mechanism along the edge of a road can improve road life by 40% when the system is installed properly. Other state agencies support this assessment. For example, the Maine DOT has observed that for an additional 20% increase in initial construction costs, proper drainage can double the expected useful life of a road. Studies by the University of Maine have quantified these observations with respect to actual soil permeability of various road bases throughout Maine. The University of Maine studies concluded that roads constructed with as little as 4% fines within the base and subbase courses drained at very slow rates, only two feet per day. This means that if a road, such as one observed in the study, had water traveling a typical distance of 20 feet, that is, 2 feet downwardly and 18 feet horizontally to a ditch or drain at the road's edge, it would take ten days for the road to drain, even if no additional fluids entered that same section of the road.
Thus, the rate at which water and other fluids are transported away from the various layers or levels of a paved surface is a critical element in its useful life. As can be easily seen, premature pavement failure due to inadequate drainage is an extremely serious and costly problem affecting the transportation infrastructure of North America and other areas. Indeed, Cedergren reported that 212 billion dollars U.S. was spent in 1991 on repairing highway deficiencies that were largely a result of poor drainage.
In one conventional method of approaching these drainage problems, an Open Graded Base Course, or “OGBC,” drainable layer formed of natural stone and aggregate materials is installed beneath a roadway or other paved structure during its construction in an attempt to positively control fluids and dissipate pore pressures which commonly accumulate under large pavement structures. Typically, an OGBC-drainable pavement includes a layer of asphalt or concrete surface pavement, a permeable base, a separate filter layer, the sub-grade, and an edge drain. In theory, an OGBC drainable pavement provides a fluid-permeable zone beneath the pavement surface in order to alleviate the hydraulic problems attendant to poor drainage. On the other hand, the optimal performance of a pavement system is achieved by preventing water from entering the pavement and removing any water that does enter by means of a well-designed subsurface drainage system.
An OGBC is intended to be a porous drainage media that is capable of receiving fluids from the points of entry and then transporting them to designated discharge points in a timely manner. According to the FHWA, a typical OGBC permeable base is estimated to have a minimum permeability of 1,000 lineal feet per day. A permeability in this range will allow for drainage of the overlying pavement to occur within a few hours and thus would be considered as “excellent drainage” as defined by AASHTO. Because OGBC is installed as a highly porous and permeable system underneath an entire pavement section, it affords drainage to fluids regardless of their points of entry. For these reasons, OGBC has been viewed in the field as having acceptable parameters of fluid interception and drainage with respect to pavement systems.
OGBC is typically produced from stone that has been mined from quarries. A main distinguishing characteristic of OGBC materials is that they are usually delivered to work sites having a fairly uniform gradation per the specifications of the project engineer. Typically, project engineers use published standards for OGBC available from AASHTO, the Federal Highway Administration, or their resident state's department of transportation. Theoretically, uniform gradation of OGBC materials typically creates voids of desired and predictable dimension between them when they are in place. Thus, desired flow rates through both vertical and horizontal planes of the OGBC can be increased or decreased somewhat predictably by selecting appropriate size distributions of the particulate material.
Nonetheless, there are many disadvantages in OGBC drainage systems that appear to be caused by the lack of mechanical and dimensional stability provided by using uniform size gradations of stone. Although such gradations create interconnecting void spaces or holes with the aggregate for the purpose of receiving and transmitting fluid, OGBC by its very nature is susceptible to unacceptable amounts of lateral movement when exposed to shear stresses caused by typical traffic loading. This condition necessitates the need to chemically bond OGBC particulate materials to one another with cementitious or bituminous materials. The use of such bonding materials serves not only to increase costs, but to actually reduce the volume and extent of void space that remains within the OGBC. Thus, by addressing the problem of lateral stress, the void space required for sufficient drainage in an OGBC is reduced to unacceptable levels. Other disadvantages pertain to the additional elements that are required in an OGBC installation. Typically, a well graded granular or geotextile filter layer is needed above the OGBC in order to prevent contamination of the OGBC from the migration of sub-grade fines. This extra filter layer further increases the costs of the roadway construction.
Although an OGBC's interconnected void spaces may afford an acceptable level of drainage for some applications, the use of an OGBC conflicts with many established road pavement design practices. This is the case because roadways designed for long-term use often require the elimination of void spaces in order to obtain strength, reduce the movement of particles, sand and aggregate, and thereby increase the load-carrying capacity of the paved surface. Furthermore, unacceptably high construction costs are sometimes incurred when using an OGBC because of the need for precision and extensive on-site quality control in order to increase the chances that a high-flow OGBC system will last for the life of the overlying paved surface.
Another particular problem with the use OGBC's for drainage relates to their long-term performance. It is not uncommon to find distress in some OGBC systems after only a few years of apparently satisfactory service. Initial indications are that the drainage from the system has slowed and that the pavement and one or more base layers are moving with respect to one another, resulting in loss of sufficient support to overlying pavement layers. Some researchers and practitioners have suggested that the failure of an open-graded base course as a drainage layer is far more detrimental to the stability of a paved surface then the presence of a fluid-saturated dense-graded base course. For this and related reasons, current concerns now focus on the long-term stability and hydraulic conductivity of the open-graded bases and their effect on pavement performance.
The hydraulic conductivity of OGBC's over time is susceptible to the deleterious clogging effects of the upward migration of subgrade soil particles into the layer, as well as from the infiltration of fine particles from fractures in the pavement surface. While there is still a need to determine the optimum balance between stability and hydraulic conductivity for the least cost, equally important is the need to identify construction methods and materials for maintaining the initial stability and hydraulic characteristics of an OGBC over time.
Yet another problem with the OGBC is that quality aggregate is not always available or, if available, at uneconomically or prohibitively high costs. There is therefore a need for a drainage system that utilizes components which can be engineered and manufactured offsite, which provide equivalent or superior flow to OGBC's and that can be integrated economically within a large paved structure to provide efficient and cost-effective drainage for the structure, while also providing sufficient dimensional, mechanical and hydraulic capability.
In general, geosynthetics are manufactured from polymeric materials, typically by extrusion, as substantially planar, sheet-like, or cuspidated products. Geosynthetics are usually made in large scale, e.g., several meters in width and many meters in length, so that they are easily adaptable to large-scale construction and landscaping uses. Many geosynthetics are formed to initially have a substantially planar configuration. Some geosynthetics, even though they are initially planar, are flexible or fabric-like and therefore conform easily to uneven or rolling surfaces. Some geosynthetics are manufactured to be less flexible, but to possess great tensile strength and resistance to stretching or great resistance to compression. Certain types of geosynthetic materials are used to reinforce large man-made structures, particularly those made of earthen materials such as gravel, sand and soil. In such uses, one purpose of using the geosynthetic is that of holding the earthen components together by providing a latticework or meshwork whose elements have a high resistance to stretching. By positioning a particular geosynthetic integral to gravel, sand and soil, that is with the gravel, sand and soil resident within the interstices of the geosynthetic, unwanted movement of the earthen components is minimized or eliminated.
Most geosynthetic materials, whether of the latticework type or of the fabric type, allow water to pass through them to some extent and thus into or through the material within which the geosynthetic is integrally positioned. Thus, geosynthetic materials and related geotechnical engineering materials are used as integral part of manmade structures or systems in order to stabilize their salient dimensions.
Until recently, the only geosynthetic materials available for pavement drainage were exclusively limited to drains at the edge or shoulder of a roadway. These edge-drain systems are commonly located within a covered trench dug along the shoulder of the roadway during its construction. Conventional edge drain geosynthetics, however, cannot withstand the repeated dynamic loads that are present directly beneath pavement surfaces.
The present invention thus offers a range of synthetic void-maintaining laminate products which overcome the many deficiencies of the OGBC. The present invention relates generally to synthetic void-maintaining structures with high permittivity and high transmissivity that are capable of extending the life of pavement and other large structures by removing undesirable fluids. The present invention includes a myriad of high-flow void-maintaining membrane laminates (“VMML's”) which possess desirable properties that make them capable of being a suitable partial, or full, replacement for conventional road-building materials such as OGBC's.
The preferred embodiments of the present invention of high throughput void-maintaining laminates overcome the previously mentioned disadvantages by providing a plurality of interconnected voids of great mechanical and dimensional stability while simultaneously providing sufficient horizontal flow to perform in accordance with “Good to Excellent” drainage when assessed with AASHTO definitions. These performance attributes are one desirable aspect of the present VMML systems because they eliminate many of the problems associated with fluids underlying large structures that are not resolved by conventional OGBC systems or by other geosynthetic products. By reducing or eliminating these problems, VMML systems extend the useful life of the overlying structure.
In accordance with other aspects of the present invention, VMML's can be positioned in a roadway to maximize their effectiveness, for example, directly beneath the pavement surface, immediately beneath the base course, or directly above a sub-grade if a sub-base is not present.
VMML's according to the invention can be made in large pieces, for example, several meters wide and many meters long. Moreover, for convenience in installation, VMML's may be installed in portions which are interconnected such that the interconnecting voids are of sufficient dimension that the water from a large structure such as a roadway can move freely through the VMML and can be connected to drain means such as a perforated pipe, ditch, or culvert adjacent to the pavement structure.
VMML's of the present invention can be fabricated into panels of various lengths and widths by using a means to weld, tie or sew VMML sections to one another to form one or more continuous VMML pathways underneath construction soils and pavement. Typically, a VMML of the present invention is positioned so that it is installed beneath pavement and above the natural soil native to the construction site. Also typically, the present VMML's reduce the distance to drain from the horizontal plane governed by the slope to the vertical distance between the SDBC and the fluid entry point.